[XPERIA X10 Q&A, Help & Troubleshooting] Dr Goodvibes:
Does S1 Tool look like the following with an uppercase 'R' in R4A024 on the line LOADER VERSION: "R4A024"

If the the 'r' is lower case the bootloader is unlocked.

Welcome to S1 tool.

RUNNING S1_EROM VER "R8A031"

SOFTWARE AID: 0001

LOADER AID: 0001

FLASH ID: "002C/00B3"

LOADER VERSION: "R4A024"

MODEL (from GDFS): X10i

SOFTWARE VERSION: 1227-4612_3.0.1.G.0.75

CUSTOM VERSION: 1241-1846_R1C

FILESYSTEM VERSION: WORLD-1-4-8_3.0.1.G.0.75

SERIAL NO: CB53KEEPUMUM

SEMC SIMLOCK CERTIFICATE

Elapsed:55 secs.​

When using FlashTool to unlock the boot loader you should see something like the following logged output as it unlocks the boot loader.

20/015/2012 16:15:44 - INFO - Device connected with USB debugging off
20/015/2012 16:15:44 - INFO - For 2011 devices line, be sure you are not in MTP mode
20/015/2012 16:15:44 - INFO - Device connected with USB debugging on
20/015/2012 16:15:45 - INFO - Connected device : X10
20/015/2012 16:15:45 - INFO - Installed version of busybox : BusyBox v1.19.0 (2011-08-14 23:46:58 CDT) multi-call binary.
20/015/2012 16:15:45 - INFO - Android version : 2.3.3 / kernel version : 2.6.29-00054-g5f01537
20/015/2012 16:15:46 - INFO - Remounting system read-write
20/015/2012 16:15:46 - INFO - Installing toolbox to device...
20/015/2012 16:15:46 - INFO - Pushing C:\Program Files\Flashtools 0.8.6\Flashtool\custom\root\ftkit.tar to /data/local/tmp
20/015/2012 16:15:46 - INFO - Running installftkit as root thru sysrun
20/015/2012 16:15:47 - INFO - Root Access Allowed
20/016/2012 16:16:22 - INFO - Please connect your device into flashmode.
20/016/2012 16:16:27 - INFO - Device disconnected
20/017/2012 16:17:22 - INFO - Device connected in flash mode
20/017/2012 16:17:22 - INFO - Opening device for R/W
20/017/2012 16:17:22 - INFO - Reading device information
20/017/2012 16:17:22 - INFO - Phone ready for flashmode operations.
20/017/2012 16:17:22 - INFO - Flashing loader
[COLOR="Red"]20/017/2012 16:17:24 - INFO - Loader : S1_Loader_Root_f851 - Version : R4A024 / Bootloader status : UNROOTABLE[/COLOR]
20/017/2012 16:17:24 - INFO - Start Reading property
20/017/2012 16:17:24 - INFO - Reading TA finished.
20/017/2012 16:17:24 - INFO - Ending flash session
20/017/2012 16:17:24 - INFO - Found semcSL
20/017/2012 16:17:24 - INFO - You can now unplug and turn on device, then plug it again
20/017/2012 16:17:24 - INFO - Waiting for device
20/019/2012 16:19:39 - INFO - Device connected with USB debugging on
20/019/2012 16:19:40 - INFO - Pushing C:\Program Files\Flashtools 0.8.6\Flashtool\custom\features\UnlockBL\files\qsd8250\qsd8250bootwrite_semcSL to /data/local/tmp
20/019/2012 16:19:40 - INFO - Pushing C:\Program Files\Flashtools 0.8.6\Flashtool\custom\features\UnlockBL\files\qsd8250\mapper_2.6.29-00054-g5f01537.ko to /data/local/tmp
20/019/2012 16:19:40 - INFO - Pushing C:\Program Files\Flashtools 0.8.6\Flashtool\custom\features\UnlockBL\files\qsd8250\fixPart to /data/local/tmp
20/019/2012 16:19:40 - INFO - Running runfixPart as root thru sysrun
20/019/2012 16:19:41 - INFO - Successfully applied fixPart. Rebooting
20/019/2012 16:19:41 - INFO - Waiting for device
20/019/2012 16:19:42 - INFO - Device disconnected
20/019/2012 16:19:53 - INFO - Device connected with USB debugging off
20/019/2012 16:19:53 - INFO - For 2011 devices line, be sure you are not in MTP mode
20/019/2012 16:19:57 - INFO - Device connected with USB debugging on
20/020/2012 16:20:10 - INFO - Running runbootwrite as root thru sysrun
20/020/2012 16:20:11 - INFO - Successfully applied unlock. Rebooting
20/020/2012 16:20:11 - INFO - Running runClean as root thru sysrun
20/020/2012 16:20:13 - INFO - Device disconnected
20/020/2012 16:20:24 - INFO - Device connected with USB debugging off
20/020/2012 16:20:24 - INFO - For 2011 devices line, be sure you are not in MTP mode
20/020/2012 16:20:33 - INFO - Device disconnected
20/020/2012 16:20:36 - INFO - Device connected with USB debugging on
20/020/2012 16:20:36 - INFO - Connected device : X10
20/020/2012 16:20:36 - INFO - Installed version of busybox : BusyBox v1.19.0 (2011-08-14 23:46:58 CDT) multi-call binary.
20/020/2012 16:20:36 - INFO - Android version : 2.3.3 / kernel version : 2.6.29-00054-g5f01537
20/020/2012 16:20:48 - INFO - Root Access Allowed
20/023/2012 16:23:12 - INFO - Device disconnected
20/024/2012 16:24:10 - INFO - Log written to C:\Program Files\Flashtools 0.8.6\Flashtool\flashtool_2012-12-19_04-10-47.log

The line highlighted in red in my example of Flashtool unlocking the boot loader scared me, but unlocked the bootloader anyway. :)

Tutorial: Rooting, Bootloader unlocking & updating